1. Field of the Invention
This invention relates generally to nuclear reactor fuel element support grids and more particularly to support grids that include mixing vanes.
2. Background Information
Fuel assemblies for nuclear reactors are generally formed from an array of elongated fuel elements or rods maintained in a laterally spaced relationship by a skeletal support structure, including a plurality of longitudinally spaced support grids, a lower end fitting, and an upper end fitting. The fuel assembly skeleton also includes guide tubes and instrumentation thimbles, which are elongated tubular members symmetrically interspersed among and positioned coextensive with the fuel element locations. The guide tubes and instrumentation thimbles are fixedly connected to the support grids to provide the structural coupling between the other skeletal members. The support grids each define an array of fuel rod support compartments or cells and have a perimeter that is configured in one of a number of alternate geometrical shapes that is dictated by the reactor core design. Nuclear fuel grids for commercial pressurized water reactors employing square fuel assemblies can typically have between 14 and 17 cells per side. Other polygonal array designs are also employed, such as the hexagonal array illustrated in U.S. Pat. No. 5,303,276, issued Apr. 12, 1994.
Each interior lattice forming member is slotted over one half of its width along its lines of intersection with the other grid forming members of the array. The members are assembled and interlocked at the lines of intersection with the slot in one member fitting into the opposing slot in the crossing member in an xe2x80x9cegg-cratexe2x80x9d fashion. This egg-crate design provides a good strength to weight ratio without severely impeding the flow of coolant that passes through the grid in an operating nuclear reactor. The lattice-forming members typically include projecting springs and dimples for engaging and supporting the fuel elements within some of the grid compartments. The springs provide axial, lateral and rotational restraint against fuel rod motion during reactor operation under the force of coolant flow, during seismic disturbances, or in the event of external impact. These spacer grids also act as lateral guides during insertion and withdrawal of the fuel assemblies from the reactor.
One of the operating limitations on current reactors is established by the onset of film boiling on the surfaces of the fuel elements. The phenomenon is commonly referred to as departure from nuclear boiling (DNB) and is affected by the fuel element spacing, system pressure, heat flux, coolant enthalpy and coolant velocity. When DNB is experienced, there is a rapid rise in temperature of the fuel element due to the reduced heat transfer that occurs under these conditions as a result of the gaseous film that forms on portions of the fuel element surface, which can ultimately result in failure of the fuel element if it was to continue. Therefore, in order to maintain a factor of safety, nuclear reactors must be operated at a heat flux level somewhat lower than that at which DNB occurs. This margin is commonly referred to as the xe2x80x9cthermal margin.xe2x80x9d
Nuclear reactors normally have regions in the core that have a higher neutron flux and power density than other regions. The variation in flux and power density can be caused by a number of factors, one of which is the presence of control rod channels in the core. When the control rods are withdrawn, these channels are filled with coolant, a moderator, which increases the local moderating capacity and thereby increases the power generated in the adjoining fuel. In these regions of high power density known as hot channels, there is a higher rate of enthalpy rise than in other channels. These hot channels set the maximum operating conditions for the reactor and limit the amount of power that can be generated, since it is in these channels that the critical thermal margin is first reached.
The prior art has attempted to reduce the variation in power density across the core and thus increase the DNB performance by providing coolant flow deflector vanes as an integral part of the fuel support grids. The vanes improve performance by increasing heat transfer between the fuel rods and the coolant downstream of the vane locations. This approach to improving performance has met with varying degrees of success depending on the vane design and the design of other grid components, which can impact the effectiveness of the vanes. To maximize the benefit of the vanes, the size, shape, bend angle, and location of the vanes must be optimized. The vanes are especially beneficial in the regions adjoining the hot channels, which are the fuel element positions adjacent to the control rod guide tube locations. It is also desirable to streamline the remaining grid components, i.e., the lattice straps, including the springs, dimples and welds, to reduce the turbulence generated in the vicinity of the vanes. Other objectives in optimizing fuel grid designs include minimizing grid pressure drop and maximizing grid load carrying strength.
In the past, nuclear fuel grid designers have mostly employed uniform coolant mixing vane patterns over the entire support grid structure. Another prior art design has used a mirror image pattern in adjacent halves of the grid, 180xc2x0 degrees out of phase. Some of these designs have experienced coolant flow induced vibrations, which can cause fretting of the fuel rods and affect their long term performance. Accordingly, an improved grid structure is desired that improves DNB performance and reduces vibrations resulting from the hydraulic turbulence generated by the grid mixing vanes. It is a further object of this invention to provide such a structure that exhibits a minimum of pressure drop and improved load carrying strength.
The structure of this invention overcomes some of the difficulties experienced in using prior art nuclear support grid designs by establishing helical coolant flow patterns in three or more symmetrical regions across the grid, that are hydraulically balanced across the grid""s center. The perimeter of the grid is shaped as an equilateral polygon and the symmetrical regions are bordered by the perimeter and lines extending between the mid point of the perimeter segments and the center of the grid. The interior of the grid is formed from a lattice whose members define the cells through which the fuel elements, control rod guide tubes, and instrumentation thimbles are supported. In one embodiment the coolant mixing vanes extend from at least some of the upper walls of the cells supporting the fuel elements. Aside from their orientation, the mixing vane pattern in each region is identical. The orientation of the pattern from region to region is rotated about the center of the grid xe2x80x9cNxe2x80x9d degrees, where xe2x80x9cNxe2x80x9d equals 360 divided by the number of segments that make up the grid perimeter.
In the preferred embodiment, the walls of the cells that surround the control rod guide tubes and instrument thimbles do not support mixing vanes and are embossed at their mid point between intersections with adjacent walls, along their height, with a concave notch that substantially matches the outside curvature of the control rod guide tubes and instrumentation thimbles, respectively. The embossed locations permit the use of tube and thimble diameters that are larger than the nominal width of the cells. Each cell is welded at a location intermediate its upper and lower ends at the intersection of its lattice straps, to improve its crush resistant strength.
Thus, the combination of rotatable and symmetrical features of the coolant mixing vane pattern produces balanced hydraulic forces acting on the fuel assembly members that enhance the support grid""s anti-vibration properties. In addition, the structural arrangement of this invention improves the strength of the grid while accommodating control rod guide tubes and instrumentation thimbles of a larger diameter than the fuel elements.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.